Causes of 2022 Pakistan flooding and its linkage with China and Europe heatwaves

Pakistan experienced a sequence of unusually intense monsoon rainfall surges from early July to late August 2022, causing flooding over one-third of the country, displacing over 30 million people, and leading to over 1000 deaths and more than USD 30 billion in losses. Accumulated rainfall was about four standard deviations above the climatological mean and approximately double that of the 2010 flooding event. Simultaneously, central China and Europe endured persistent heatwaves severely affecting agriculture and power supply, suggesting possible linkages among these extremes. The year 2022 was during a triple-dip La Niña beginning in 2020. Prior work shows La Niña summers often feature a strong Western North Pacific Subtropical High (WNPSH) and strengthened Indian monsoon flow. While Niño4 SST was unusually low in 2022, regressions of 850-hPa moisture flux on Niño4 and Niño3.4 showed minor differences over Pakistan, implying La Niña alone cannot fully explain the anomalous rainfall. The 2010 and 2022 events shared similarities: both La Niña years with a European blocking high and southerly monsoon surges from the Arabian Sea, and both connected via an extratropical Rossby wave-like perturbation across Eurasia. Additionally, rising sea surface temperatures in the Indian Ocean and enhanced atmospheric perturbations in recent decades could intensify rainfall and heatwaves. The study explores the physical processes behind the 2022 record rainfall, focusing on compounding factors and the linkage to concurrent heatwaves in Europe and China. The hypothesis is that anomalous La Niña-induced easterlies over the northern Indian subcontinent, enhanced southerly flow from the Arabian Sea with an upward trend, and tropical-extratropical interactions between monsoon flow and northerly surges combined to produce the flooding, with a Rossby wave-like teleconnection connecting the Pakistan flooding and China/Europe heatwaves.

The study builds on literature showing La Niña summers strengthen the WNPSH and monsoon circulations over South and East Asia and that the 2010 Pakistan floods were linked to a tropical-extratropical interaction involving European blocking and intensified monsoon flow in the western Indian Ocean. Previous work identified La Niña-related circulation as crucial in 2010 and suggested similar impacts for 2022. Studies also document rising Indian Ocean SSTs since the 1980s, enhancing moisture availability and transport, and increasing frequency, intensity, and duration of global heatwaves with warming. Reported teleconnections influencing Eurasian summer climate include the Scandinavia pattern, circumglobal teleconnection (CGT), and British-Okhotsk Corridor (BOC), though the 2022 pattern shows differences from these canonical modes. Research has also highlighted the role of atmospheric rivers from the Arabian Sea and compound events arising from synchronized factors. These prior findings frame the investigation into whether compounding influences and an emerging teleconnection pattern drove the 2022 extremes.

Data: Daily precipitation from CPC Global Unified (0.5°), monthly precipitation from GPCP v2.3 (2.5°) and CRU TS v4.07 (2.5°). ERA5 daily and monthly reanalysis (winds, geopotential height, specific humidity) regridded to 2.5°. Monthly ERSSTv5 (2°) SST. Daily NOAA interpolated OLR (2.5°). Indices and regions: Pakistan rainfall (22°–32°N, 63°–73°E), Central China T2m (25°–35°N, 90°–120°E), Northeastern Europe (various as specified). Time period primarily 1979–2022; focus on July–August. Diagnostics and analyses: - Constructed daily and monthly indices; Hovmöller diagrams of 10-m meridional wind (60°–75°E) and OLR; time series of surges. - Regressed T2m, precipitation, 500-hPa geopotential height (H500), and moisture flux fields on normalized Pakistan rainfall index. - Computed vertically integrated (surface–300 hPa) moisture flux and moisture flux convergence (MFC). - Assessed statistical significance using Pearson correlation with Fisher’s Z-transform and Student’s t-test for regression coefficients. - Wave activity flux and stationary Rossby wave diagnostics followed Takaya and Nakamura (2001), applied at 200 hPa, along with Rossby wave source and streamfunction anomalies for July and August. - Blocking identification via a 500-hPa geopotential height meridional gradient-based blocking index, BI > 0 criterion at multiple latitudinal offsets (Δ = −5, −2.5, 0, 2.5, 5). - Heatwave index defined as the number of grid points exceeding the local 95th percentile of daily 2-m temperature within 20°–40°N, 80°–120°E. - Idealized response experiments using a linear baroclinic model (LBM) with imposed diabatic heating centered over Pakistan (29°N, 68°E) in August climatological background flow to diagnose circulation responses at 200 and 850 hPa (day-30 output). - Trend analyses: linear trends in moisture flux, SST, and geopotential height; correlations between Pakistan rainfall and Arabian Sea southerly flow with and without trend removed; estimation of trend contribution to anomalous MFC. - Comparative analyses across years (2010, 2020, 1998, 1999) including SST patterns (IOD-like vs basin-wide), WNPSH strength and extent, and presence of extratropical wave activity.

- Pakistan’s July–August 2022 rainfall exceeded the 99th percentile (1979–2022), with record accumulated rainfall of about 450 mm during July 1–August 31, approximately four times the climatological average and double the 2010 record. - Three monsoon rainfall surges occurred: early–mid July, late July, and early–late August. Their contributions to July–August rainfall were ~32%, ~16%, and ~49%, respectively, exceeding 90% in total. - Large-scale circulation featured intensified southwesterlies from the Arabian Sea converging with La Niña-induced easterly anomalies over the northern Indian subcontinent; blocking highs and extreme warmth over Europe and central China. - A tropical-extratropical interaction was critical, especially in August: extratropical northerlies associated with European blocking penetrated southward and converged with warm-moist southerlies over Pakistan, favoring deep convection. - Regression and wave activity flux analyses identified a stationary Rossby wave-like teleconnection linking northeastern Europe, Pakistan, and central China. The pattern resembles but is distinct from canonical SCA, CGT, and BOC patterns. - Pakistan rainfall is significantly correlated with central China T2m (cc = 0.44, P < 0.01) and H500 (cc = 0.57, P < 0.0001), and with northeastern Europe T2m (cc = 0.33, P < 0.05) and H500 (cc = 0.28, P < 0.1). Central China T2m correlates strongly with an East Asia heatwave index (cc = 0.79, P < 0.0001); the heatwave index correlates with Pakistan rainfall (cc = 0.39, P < 0.01). - La Niña alone could not explain the 2022 extremes: Niño4 was very low, yet regressions of 850-hPa moisture flux on Niño4 vs Niño3.4 showed only minor differences over Pakistan; Pakistan rainfall was low in other La Niña summers (e.g., 1998, 1999). - The Indian Ocean Dipole did not account for the enhanced Arabian Sea southerlies in 2022: correlation between 850-hPa meridional wind over the Arabian Sea (10°–20°N, 50°–65°E) and the IOD index was 0.03 (P = 0.83). - A significant upward trend in moisture flux over the Arabian Sea and from the Bay of Bengal to the northern Indian subcontinent contributed to stronger moisture flux convergence over Pakistan; trend explains ~21% of the anomalous MFC over Pakistan in 2022. - Pakistan rainfall correlates with Arabian Sea southerly flow (cc = 0.32, P < 0.05), increasing to 0.41 (P < 0.001) when including linear trend contribution. - Numerical experiments suggest positive feedback: convection-induced diabatic heating over Pakistan can accelerate southerly flow from the Arabian Sea and enhance anticyclonic anomalies over China, reinforcing both rainfall and heatwaves. - The 2022 event was an intensified manifestation of 2010, sharing compounding factors but occurring under stronger upward trends in both tropics and extratropics. - The teleconnection pattern became more prominent in 2022 and may represent an emerging Eurasian wave-like structure distinct from known patterns.

The findings address the research question by demonstrating that the 2022 Pakistan record rainfall arose from the concurrence of multiple large-scale forcings: La Niña-enhanced easterlies associated with a westward-extended WNPSH, enhanced and trend-amplified southerlies from the Arabian Sea, and a tropical-extratropical interaction with southward-penetrating northerlies tied to European blocking. These interacting components created strong low-level moisture convergence and instability over Pakistan, producing three surges culminating in a prolonged August event. The study also establishes a robust linkage between Pakistan flooding and heatwaves in Europe and central China via a stationary Rossby wave-like teleconnection spanning Eurasia. Regression and wave activity flux analyses support an extratropical origin of the anomaly pattern, with diabatic heating over Pakistan potentially reinforcing downstream anticyclones over China, implying mutual enhancement between wet and dry extremes. The results underscore that La Niña alone was insufficient; only when multiple drivers synchronize do such extremes emerge, explaining their rarity. The distinct spatial structure compared with SCA, CGT, and BOC suggests an emerging teleconnection with growing influence. These insights are relevant for understanding and anticipating concurrent extremes across Eurasia under ongoing warming and highlight the importance of background trends in moisture transport and circulation.

This study documents that the 2022 Pakistan flood was driven by compound influences: La Niña-induced low-level easterlies and strengthened WNPSH, enhanced southerly moisture transport from the Arabian Sea amplified by recent trends, and a tropical-extratropical interaction linked to European blocking. A stationary Rossby wave-like teleconnection connected Pakistan flooding with heatwaves in central China and northeastern Europe, differing from canonical SCA, CGT, and BOC patterns and potentially representing an emerging mode. Positive feedbacks via diabatic heating further intensified the event. The 2022 flood represents an intensified counterpart of the 2010 event within a warming climate context, where background trends in moisture flux and geopotential height contributed notably to extremity. Future research should clarify causal feedbacks between Pakistan rainfall and China heatwaves, determine the dynamics and predictability of the emerging wave-like teleconnection, quantify how continued warming may alter Arabian Sea southwesterlies and compound-event likelihood, and employ targeted numerical experiments to isolate process contributions.

- Causality between Pakistan rainfall and the central China heatwave is not fully resolved; feedback pathways remain uncertain. - The identified Rossby wave-like teleconnection differs from known patterns (SCA, CGT, BOC), but its precise nature and stationarity are not conclusively established. - Idealized linear baroclinic model experiments provide suggestive responses but may not capture full nonlinear dynamics or land-atmosphere coupling. - Attribution of trend contributions (e.g., ~21% of anomalous MFC) carries uncertainty due to linear trend assumptions and dataset limitations. - Event-specific conclusions may not generalize across all La Niña summers or different background states without further targeted studies.